1. Field of the Invention
The present invention relates to a method of controlling communications in a communication system comprising at least three communication control units connected to a single transmission line, and more particularly to a method of controlling communications in such a communication system in which at least one set of communication control units is in an exclusive communication mode.
2. Description of the Related Art
One conventional method of controlling communications will be described below with reference to FIGS. 1 through 3 of the accompanying drawings.
FIG. 1 shows in block form a conventional communication control apparatus 1. FIG. 2 shows in block form a communication system comprising three communication control apparatus A.sub.1, A.sub.2, B, each identical to the conventional communication control apparatus 1, which are connected to a single transmission line 20. FIG. 3 shows a sequence of operation in which data is transmitted from communication control apparatus B to communication control apparatus A.sub.1.
As shown in FIG. 1, communication control apparatus 1 comprises a transmitter/receiver 2 for transmitting a communication frame composed of message data and a transmission control signal by the transmission line 20 to another communication control apparatus according to a predetermined communication protocol and for receiving a message frame transmitted from the other communication control apparatus, an address memory 3 for storing address information of all communication control apparatus that are connected to the transmission line 20, and a controller 4 for controlling operation of the transmitter/receiver 2 and the address memory 3. In FIG. 2, three communication control apparatus A.sub.1, A.sub.2, B, each identical to communication control apparatus 1, are connected to the transmission line 20. However, more communication control apparatus may be connected to the transmission line 20.
Now, it is assumed that information is to be continuously transmitted from one of the communication control apparatus, for example, communication control apparatus A.sub.1, to another communication control apparatus, for example, communication control apparatus A.sub.2. To prevent another communication control apparatus from intervening in the communication between communication control apparatus A.sub.1 and A.sub.2, communication control apparatus A.sub.1 first transmits a communication frame indicative of an exclusive communication mode request to communication control apparatus A.sub.2, as shown in FIG. 3. If communication control apparatus A.sub.2 has properly received the communication frame and it is possible to effect exclusive communications between the communication control apparatus A.sub.1 and A.sub.2, communication control apparatus A.sub.2 sends an acknowledgment (ACK) signal to communication control apparatus A.sub.1. After having received the ACK signal, communication control apparatus A.sub.1 continuously transmits a communication frame to communication control apparatus A.sub.2. When the transmission of the communication frame is finished, communication control apparatus A.sub.1 transmits a communication frame for releasing the exclusive communication mode to communication control apparatus A.sub.2. The exclusive communication mode between communication control apparatus A.sub.1 and A.sub.2 continues for the period of time after communication control apparatus A.sub.2 sends the ACK signal until it receives the communication frame for releasing the exclusive communication mode. If during this period communication control apparatus B, which is not in an exclusive communication mode, transmits a communication frame to communication control apparatus A.sub.2, communication control apparatus A.sub.2 returns a negative acknowledgment (NAK) signal to communication control apparatus B as communication control apparatus A.sub.2 cannot receive the communication frame from communication control apparatus B.
As described above, the exclusive communication mode continues until communication control apparatus A.sub.2 receives the communication frame for releasing the exclusive communication mode which is transmitted from communication control apparatus A.sub.1, which is the communication control apparatus that initiated the exclusive communication mode. Since communication control apparatus B is not supplied with the communication frame for releasing the exclusive communication mode, communication control apparatus B, which has received the NAK signal, must continuously call until exclusive communication mode is released and an ACK signal is returned.
Japanese patent laid-open publication No. 44151/1989 discloses a communication system which is designed to solve the above problem. According to the disclosed communication system, if communication control apparatus A.sub.1, which has requested an exclusive communication mode, fails to transmit a communication frame for releasing the exclusive communication mode during a predetermined period of time after having transmitted a communication frame, communication control apparatus A.sub.2, which has been requested to enter the exclusive communication mode, releases the exclusive communication mode itself, thus shortening the period of time occupied for the exclusive communication mode.
The disclosed communication system, however, still suffers a drawback in that while a communication control apparatus is in an exclusive communication mode for a long period of time or in a simultaneous broadcasting communication mode in which it transmits a communication frame without requesting an exclusive communication mode, another communication control apparatus which is not in the exclusive communication mode cannot access the communication control apparatus in the exclusive communication mode.